1. Field of the Invention
The present invention relates to a ball screw, comprising a spindle element which has a first groove arrangement on an outer circumferential surface, a nut element which surrounds the spindle element and has a second groove arrangement on an inner circumferential surface, and a plurality of balls, at least some of which engage into the first groove arrangement and the second grove arrangement, the nut element being rotatable about an axis of rotation with respect to the spindle element, and, during the rotation of the nut element about the axis of rotation with respect to the spindle element, the balls which engage into the first groove arrangement and the second groove arrangement rolling in the first and the second groove arrangement, while at the same time rotating about respective ball axes of rotation.
2. Description of the Related Art
Ball screws of this type are employed in many areas of use in which a rotational movement is to be converted into a rectilinear displacement movement. For example, movement conversion arrangements of this type are employed in automatically controlled machine tools, and use as a clutch disengager may also be envisaged.
A basic requirement of ball screws of this type, which ultimately serve for converting a rotational movement into a displacement movement, is that, during the conversion of movement, as little energy as possible is dissipated as frictional energy or the like and therefore, after conversion, is no longer available on the output side. There is a problem, here, in that the various ball elements come to bear or roll on regions of the spindle element and of the nut element which have a different radius with respect to the axis of rotation. For example, in terms of a single revolution of a respective ball about the ball axis of rotation, this means that, in this case, the ball would have to cover a greater distance along the bearing region on the second groove arrangement than on the corresponding surface of the first groove arrangement. This necessitates a partially slipping movement of the balls in the bearing region in at least one of the groove arrangements and consequently the introduction of a sliding movement with a corresponding loss due to sliding friction.
The object of the present invention is to provide a ball screw in which the conversion of movement into rotational movement into displacement movement takes place with a reduced energy loss.
This object is achieved, according to the invention, by means of a ball screw, comprising a spindle element which has a first groove arrangement on an outer circumferential surface, a nut element which surrounds the spindle element and has a second groove arrangement on an inner circumferential surface, and a plurality of balls, at least some of which engage into the first groove arrangement and the second groove arrangement, the nut element being rotatable about an axis of rotation with respect to the spindle element, and, during the rotation of the nut element about the axis of rotation with respect to the spindle element, the balls which engage into the first groove arrangement and the second groove arrangement rolling in the first and second groove arrangement, while at the same time rotating about respective ball axes of rotation.
There is also provision, at the same time, with respect to the ball axis of rotation, for an effective ball rolling radius with which the balls roll in the first groove arrangement to be smaller than an effective ball rolling radius with which the balls roll in the second groove arrangement.
The provision of different effective ball rolling radii in the region of interaction with the various groove arrangements ensures that a larger effective ball rolling radius is provided at that groove arrangement at which, because of its greater radial distance from the axis of rotation, a ball element has to cover a greater distance during the rolling movement, so that, for example, again in terms of a single ball revolution about the respective ball axis of rotation, this compensates for the fact that a longer rolling distance has to be covered on the nut element located radially further outward. In spite of an identical rotational speed of the balls about the respective ball axes of rotation, therefore, a different rolling distance can be provided on the various bearing regions during a revolution of the balls.
For example, the configuration of the ball screw according to the invention may be such that the balls can roll on a first rolling-surface region of the first groove arrangement and a second rolling-surface region of the second groove arrangement, while at the same time rotating about the respective ball axis of rotation, and that, at least during the rolling movement, the balls are in contact via the first rolling-surface region with a first ball-surface region which is at a shorter distance from the ball axis of rotation than a second ball-surface region with which, at least during the rolling movement, the balls are in contact with the second rolling-surface region. This may be achieved, for example, in that the first groove arrangement has a groove cross-sectional profile with a first surface radius of curvature in the region of the first rolling-surface region, in that the second groove arrangement has a groove cross-sectional profile with a second surface radius of curvature in the region of the second rolling-surface region, and in that the first surface radius of curvature is smaller than the second surface radius of curvature. In order to obtain defined interaction between the balls and the groove arrangements and, in particular, also to make it possible for the ball to be inserted easily into the groove arrangements, it is proposed that the first groove arrangement and the second groove arrangement have a respective groove cross-sectional profile with a surface radius of curvature decreasing from a groove-aperture region toward a groove bottom.
If there is provision for the surface radius of curvature of the first groove arrangement and/or of the second groove arrangement to be smaller in the region of the groove bottom than the radius of the balls, this ensures that the balls cannot come into contact with the groove bottom. There is thus produced between the widest ball regions located in the groove arrangements and the groove bottom an interspace, in which bearing contact cannot occur and which may therefore, in particular, also serve for the reception of impurities.
The ball screw according to the invention may be designed, for example, in such a way that the first groove arrangement has at least one thread flight, preferably with a plurality of turns, and that the second groove arrangement has, assigned to each thread flight of the first groove arrangement, at least one second thread flight, the at least one second thread flight preferably having at least one turn, the turn end regions of which are connected to one another via a ball return portion.